Carbonate minerals are common oil-bearing formations, and usually consist of predominantly limestone (calcium carbonate) or dolomite (calcium magnesium carbonate).
More specifically, the thermal recovery of bitumen or heavy oil requires some manner of heating of the reservoir. When hot water, either injected as steam or from heating of naturally present water, is in contact with heavy oil or bitumen, chemical reactions are known to occur which, among other products, cause the liberation of carbon dioxide and hydrogen sulphide.
The carbon dioxide so formed will normally be dissolved in the water, and is thus available for attack on the carbonate, causing the formation of free calcium and magnesium ions. The reactions are
for limestoneCO2+H2O+CaCO3→Ca2++2HCO3−
and for dolomite2CO2+2H2O+CaMg(CO3)2→Ca2++Mg2++4HCO3−
These reactions, which are responsible for the formation of limestone caves in nature, initially have the effect of opening the pore space of the carbonate rock, thus improving fluid flow. However, near the producing wellbore, the pressure is normally reduced as a consequence of the production technique used. This reduced pressure causes the at least partial reversal of the above reactions. Thus, solid carbonate material that is initially dissolved and removed at some greater distance from the production well, is re-precipitated and thus partially or wholly fills the original pore space near the production well. This effect inhibits the production of oil.
No patents or published applications at present relate to the recovery of bitumen or heavy oil from carbonate reservoirs by thermal methods that address the effect described above.
However, a number of tangentially relevant patents or applications have been published for recovery of bitumen in reservoirs that consist of unconsolidated sands.
Canadian Patent No. 1,130,201 (Butler) teaches a thermal method for recovering highly viscous oil from bitumen deposit in unconsolidated sand by means of Steam Assisted Gravity Drainage (hereinafter referred to as SAGD). The method consists of drilling two long horizontal wells, parallel and in the same direction, with one located several meters above the other. Steam is injected into the top well, thermal communication is established between the two wells, and oil and water drain continuously to the bottom well from where they are pumped to surface.
Canadian Patent No. 2,277,378 (Cyr and Coates) teaches a thermal process for recovery of viscous hydrocarbon that is operated in a similar manner as SAGD. A third parallel and co-extensive horizontal well is provided at a suitable lateral distance from the SAGD well pair described by Butler in Canadian Patent No. 1,130,201. The purpose of the third is to practice cyclic steam stimulation in such a manner as to improve the heat distribution throughout the subterranean reservoir. In the SAGD well pair, steam will tend to rise to the top of the hydrocarbon bearing structure. By cyclic steam stimulation at the third well, steam injection is alternated with oil production to achieve a more favourable heat distribution than is possible with SAGD alone.
Canadian Patent Application No. 2,591,498 (Arthur, Gittins and Chhina) teaches an extended SAGD process with a similar well configuration to U.S. Pat. No. 2,277,378 by Cyr. The purpose is likewise to access a region of bitumen which would normally be bypassed by SAGD if operated in the manner taught by Butler. The purpose here is to “access that portion of said reservoir whose hydrocarbons have not been or had not been recovered in the course of the . . . gravity controlled process”. The recovery method from the third well, referred to as an infill well, is expected to be a gravity-controlled process, though not necessarily limited to SAGD. Reference is made to injection of light hydrocarbons or gases to maintain pressure once steam injection is discontinued.
Canadian Patent Nos. 2,015,459 and 2,015,460 (Kisman) teach a technique of gas injection into a thief zone in a bitumen bearing sand. This thief zone causes an unwanted degree of lateral steam migration from the vertical wells; the gas injection prevents this unwanted lateral migration by establishing a confining pressure from outside the well pattern, so that the steam cannot escape.
Some early pilots in Northern Alberta used cyclic steam stimulation in the 1980's (UNOCAL Buffalo Creek and McLean Pilots). Limited information is available in the public domain, however, the production data are public. The pilots were abandoned as uneconomic. The production data are accessible via the AccuMap® System from IHS® (www.ihs.com).
It is, therefore, desirable to provide a process for bitumen or heavy oil recovery from carbonate reservoirs.